Zika virus causes a mild, flu-like illness in most people, but to pregnant women the dangers are potentially much worse. The virus can reduce fetal growth, cause microcephaly, an abnormally small head associated with brain damage, and even trigger a
miscarriage.

Now, a new study in mice shows that females vaccinated before pregnancy and infected with Zika virus while pregnant bear pups who show no trace of the virus. The findings offer the first evidence that an effective vaccine administered prior to pregnancy can
protect vulnerable fetuses from Zika infection and resulting injury.

"There are several vaccines in human trials right now, but to date, none of them has been shown to protect during pregnancy," said Michael S. Diamond, MD, PhD, the Herbert S. Gasser Professor of Medicine at Washington University School of Medicine in
St. Louis, and the study's co-senior author. "We tested two different vaccines, and they both provided substantial protection."

The study is published in the journal Cell.

Zika made international headlines when it was linked to an epidemic of babies born with microcephaly in Brazil. There is no specific medicine or vaccine to prevent or treat disease caused by the mosquito-borne virus.

Last year, Diamond and others developed a mouse model of Zika infection that mimics the effects of the infection in pregnant women. Using this model, Diamond, along with co-senior authors Pei-Yong Shi, PhD, of the University of Texas Medical Branch
(UTMB), and Ted Pierson, PhD, of the National Institute of Allergy and Infectious Diseases of the National Institutes of Health (NIH), evaluated the ability of two vaccines to protect fetuses whose mothers were infected during pregnancy.

One, a so-called subunit vaccine that is based on the genetic blueprint for two proteins from the virus's outer shell and is developed by Moderna Therapeutics, is already in safety testing in women who are not pregnant and men. The other, a live but weakened
form of the virus that was developed at UTMB, is being tested in animals.

As part of the study, groups of 18 to 20 female mice were vaccinated with one of the vaccines or a placebo, and some animals received a second dose of the same vaccine or placebo a month later. Three weeks later, the researchers measured antibody levels in
the mice's blood as a measure of the strength of their immune response. They found that both vaccines had elicited very high levels of neutralizing antibodies against Zika, while the placebos had not.

After the mice became pregnant, they were infected on the sixth day of pregnancy, to mimic the experience of a woman bitten by a Zika-carrying mosquito early in pregnancy.

One week after infection, the researchers measured the amount of virus in the mothers and fetuses.